It has been previously proposed that velocity of liquid flow through a pipe be detected by the transmission and reception of ultrasonic signals on the order of 0.6 to 1.2 MHz, being transmitted and received through the side walls of a pipe through which the liquid flows. A pair of transducers, one for transmission and the other for reception, are positioned on the outside of the pipe and oriented for sound transmission across the flow of the liquid at an angle with respect to the direction of flow on the order of 60.degree.. Such a system is described as operating on the Doppler principle of measurement wherein the velocity of flow (e.g., in feet per second), ##EQU1## In this equation, C is equal to the velocity of sound in the liquid (coordinately in feet per second), F.sub.1 is the frequency of transmitted ultrasonic sound, F.sub.2 is the received frequency (shifted by the Doppler shift), and .theta. is equal to the angle between the direction of flow and the direction of transmission of the signal.
Two problems have been noted with respect to this type system. One is that velocity readouts tend to be effected by depth. A second one is that difficulties are often experienced in reading relatively low values of flow, for example, on the order of 0.5 to 3 feet per second, which is particularly important in the measurement of certain types of flow, e.g., flow of sewage.
It is the object of this invention to provide an improved ultrasonic velocity measurements system for liquid flows in pipes which eliminates the stated difficulties.